1. Field of the Invention
The present invention relates to a method of transmitting control information for reverse link packet transmission in mobile communications, and more particularly, to a method of transmitting data rate control information and ACK/NAK signal via MAC(Medium Access Control) channel.
2. Discussion of the Related Art
CDAM 2000 mobile communications system has been developed to fit applications requesting voice call or continuous data transmission. As a demand for packet data services lately increases, the first generation mobile communications system of CDMA 200, 1 x EV-DO (Evolution-Data Only), has been developed to support such a demand.
The 1xEV-DO system is designed to support a packet data service on a frequency bandwidth independent from that for a voice service only. Hence, AMC (Adaptive Modulation and Coding) and HARQ(Hybrid Automatic Repeat Request) are applied to a forward link of the 1xEV-DO system. In HARQ, a baseband packet is divided into a plurality of subpackets and retransmission is requested by subpacket unit.
In reverse link of the 1xEV-DO system, a data rate is controlled via RAB (Reverse Activity Bit). Namely, in case that each terminal transmits data via reverse link, data transmission starts at a data rate of 9,600bps that is the minimum and whether to increase the data rate is then decided by itself via p-persistent test. In doing so, a probability value changing the data rate can be differently defined according to the data rate. Generally, if a current data rate is relatively high, the probability value increasing the current data rate is defined by a small value. On the contrary, if a current data rate is relatively low, the probability value increasing the current data rate is defined by a great value.
A base station checks a state of reverse link. The base station transmits RAB signal meaning ‘busy’ to terminals in common if deciding that reverse load is excessive. If there is a margin in load of reverse link, the base station does not transmit the RAB signal. Having received the RAB signal meaning ‘busy’, each of the terminals performs the p-persistent test by itself and then decides whether to lower the current data rate by one step according to the corresponding result.
MAC(Medium Access Control) channel of the related art 1xEV-DO system is a channel to transmit control signals for the data rate control and the reverse link power control to a terminal. The MAC channel consists of three sub-channels, RPC channel, DRCLock channel, and RA channel.
The RA(Reverse Activity) channel transmits RAB(Reverse link Activity Bit) stream. The MAC channel consists of BPSK modulated into a specific phase (I- or Q-phase) of a carrier by Walsh channel that orthogonally covers. Each Walsh channel shown in Equation 1 and Equation 2 is distinguished by MAC index value between 0˜63 and can be defined by unique 64—ary Walsh cover and modulation phase.Wi/264 for i=0, 2, . . . , 62   [Equation 1]W(i−1)/2+3264 for i=1, 3, . . . , 63   [Equation 2]
In this case, ‘i’ is MAC index value. In-phase (i) modulation phase is allocated to MAC channel having even MAC index value. Quardrature-phase (Q) modulation phase is allocated to MAC channel having odd MAC index value. MAC symbol Walsh cover transmits a burst of 64-chips length four times per slot. Theses bursts, which are attached to front and rear of a pilot burst of each slot, are transmitted. Symbol of each MAC channel is transmitted over one of the Walsh channels. The Walsh channel is scaled to uniformly maintain total transmit power.
FIG. 1 is a block diagram of MAC channel according to a related art.
Referring to FIG. 1, MAC channel consists of RPC bit for reverse link rate control, DRCLock channel, and RAB, in which TDM (Time Division Multiplexing) is carried out on the RPC bit, DPRCLock channel, and RAB. The MAC channel is demodulated to transmit by BPSK via I-channel (even MAC index) or Q-channel (odd MAC index) according to the MAC index.
MAC channel is defined in forward link of the current 1xEV-DO system, and various kinds of necessary information are transmitted via MAC channel for the reverse control. The MAC channel allocated to one user includes three sub-channels in forward link of the current 1xEV-DO system. Namely, it includes RPC channel used for the reverse power control, RAB channel used for the rate control of reverse link, and DRCLock channel for the base station to feed back to the terminal whether to enable to decode DRC (data rate) the corresponding terminal has transmitted. Currently, each of the entire MAC channels available in use consists of the three sub-channels.
For the data rate control of reverse link of the 1xEV-DO system, RAB is generated from the base station based on total interference amount estimated by the base station. Namely, without considering a channel status of each terminal, RAB is generated in viewpoint of the base station. Moreover, as mentioned in the foregoing description, each terminal decreases or increases the reverse link data rate according to the RAB transmitted from the base station and the and p-persistent test, whereby the entire terminals raise or lower their data rates with the same probability regardless of the channel status of each channel.
However, the reverse data rate control in the current 1xEV-DO system decides to increase or decrease the data rate without considering the channel status or data rate of the terminal. Hence, data transmission is inefficiently performed to reduce throughput thereof.